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annotate weatherlog/reader.py @ 3:d961c8a93d6b

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reader: Use floor-division for list index.
author Paul Fisher <paul@pfish.zone>
date Fri, 27 Sep 2019 21:38:23 -0400
parents 9310e2ff7e17
children 885bff085edf
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18dc6245c91a Create initial version of weather sensor reader.
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1 """A module for reading data from the actual temperature sensor.
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2
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3 You should probably assume that this is wildly thread-unsafe.
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4 """
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5
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6 import abc
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7 import datetime
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8 import time
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9 import typing as t
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10
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11 import adafruit_dht
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12 import attr
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13 import board
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14 import pytz
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15
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16
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17 def _utc_now() -> datetime.datetime:
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18 """utcnow, but timezone-aware."""
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19 return datetime.datetime.utcnow().replace(tzinfo=pytz.UTC)
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20
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21
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22 @attr.s(frozen=True, slots=True)
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23 class Reading(object):
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24 """A single reading from a temperature/humidity sensor."""
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25
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26 # The temperature, in degrees Celsius.
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27 temp_c = attr.ib(type=float)
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28
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29 # The relative humidity, in percent.
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30 rh_pct = attr.ib(type=float)
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31
1
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32 # The timestamp of the reading.
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33 timestamp = attr.ib(type=datetime.datetime, factory=_utc_now)
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34
0
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35
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36 class Reader(metaclass=abc.ABCMeta):
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37 """Interface for a thing which reads temperatures."""
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38
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39 @abc.abstractmethod
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40 def read(self) -> Reading:
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41 """Reads a value from the weather sensor."""
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42 raise NotImplementedError()
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43
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44
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45 class DHT22Reader(Reader):
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46 """A reader for the DHT22 sensor, which attempts to make good readings."""
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47
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48 def __init__(self, pin: t.Any = board.D4):
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49 """Initializes a DHT22 reader connected to the specified pin."""
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50 self.device = adafruit_dht.DHT22(pin)
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51
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52 _TEMP_C_EPSILON = 0.51
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53 _RH_PCT_EPSILON = 1.01
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54 _NEW_READING_SECS = 2.5
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55
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56 def read(self) -> Reading:
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57 """Reads a value from the sensor. This will block until done."""
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58 temps: t.List[float] = []
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59 humids: t.List[float] = []
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60 while True:
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61 temp, rh = self._read_once()
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62 if _is_reasonable_temp(temp):
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63 temps.append(temp)
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64 if _is_reasonable_rh(rh):
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65 humids.append(rh)
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66 try:
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67 return Reading(
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68 temp_c=_last_stable(temps, epsilon=self._TEMP_C_EPSILON),
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69 rh_pct=_last_stable(humids, epsilon=self._RH_PCT_EPSILON))
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70 except ValueError:
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71 # There is not yet enough data to see if the value is stable.
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72 # Wait for a new reading.
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73 time.sleep(self._NEW_READING_SECS)
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74
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75 _RETRY_WAIT_SECS = 0.5
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76
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77 def _read_once(self) -> t.Tuple[float, float]:
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78 """Tries very very hard to read a single value from the sensor."""
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79 while True:
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80 try:
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81 temp, rh = self.device.temperature, self.device.humidity
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82 if None not in {temp, rh}:
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83 return temp, rh
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84 except RuntimeError:
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85 pass # This will happen a lot. Just try again.
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86 time.sleep(self._RETRY_WAIT_SECS)
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87
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88
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89 def _last_stable(
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90 vals: t.Iterable[float],
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91 *,
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92 epsilon: float,
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93 count: int = 3) -> float:
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94 """Returns the last stable value from the given sequence.
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95
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96 The stable value is defined as the last value for which there are `count`
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97 values in the sequence within `epsilon` of some value. This is used to
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98 eliminate misread values in the sequence, usually due to timing issues.
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99 The `count` values need not be in sequence.
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100
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101 The value returned is the median value of the group (or the next-to-median
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102 in the case of an even count).
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103 """
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104 buckets: t.Dict[float, t.List[float]] = {}
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105 for v in vals:
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106 buckets.setdefault(v, [])
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107 for bucket, bucket_vals in buckets.items():
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108 if abs(v - bucket) <= epsilon:
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109 bucket_vals.append(v)
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110 if len(bucket_vals) == count:
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111 return sorted(bucket_vals)[count // 2]
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112 raise ValueError('No stable value.')
0
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113
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114
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115 def _is_reasonable_temp(temp: float) -> bool:
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116 """True if the temperature is a reasonable level for the boathouse."""
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117 return -40 <= temp <= 60
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118
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119
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120 def _is_reasonable_rh(rh: float) -> bool:
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121 """True if the relative humidity is plausible."""
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122 return 0 <= rh <= 100